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PROBLEMS
sec. 5-6 Newton's Second Law
•1Only two horizontal forces act on a 3.0 kg body that can move over a frictionless floor. One force
is 9.0 N, acting due east, and the other is 8.0 N, acting 62° north of west. What is the magnitude of
the body's acceleration?
Answer:
2.9 m/s2
•2Two horizontal forces act on a 2.0 kg chopping block that can slide over a frictionless kitchen
counter, which lies in an xy plane. One force is
. Find the
acceleration of the chopping block in unit-vector notation when the other force is (a)
, (b)
, and (c)
.
2
•3If the 1 kg standard body has an acceleration of 2.00 m/s at 20.0° to the positive direction of an x
axis, what are (a) the x component and (b) the y component of the net force acting on the body, and
(c) what is the net force in unit-vector notation?
Answer:
(a) 1.88 N; (b) 0.684 N; (c) (1.88 N) + (0.684 N)
••4While two forces act on it, a particle is to move at the constant velocity
. One of the forces is
. What is the other
force?
••5
Three astronauts, propelled by jet backpacks, push and guide a 120 kg asteroid toward a
processing dock, exerting the forces shown in Fig. 5-29, with F1 = 32 N, F2 = 55 N, F3 = 41 N, θ1
= 30°, and θ3 = 60°. What is the asteroid's acceleration (a) in unit-vector notation and as (b) a
magnitude and (c) a direction relative to the positive direction of the x axis?
Figure 5-29Problem 5.
Answer:
(a) (0.86 m/s2) - (0.16 m/s2) ; (b) 0.88 m/s2; (c) - 11°
••6In a two-dimensional tug-of-war, Alex, Betty, and Charles pull horizontally on an automobile tire
at the angles shown in the overhead view of Fig. 5-30. The tire remains stationary in spite of the
three pulls. Alex pulls with force
of magnitude 220 N, and Charles pulls with force
magnitude 170 N. Note that the direction of
force
of
is not given. What is the magnitude of Betty's
?
Figure 5-30Problem 6.
••7
There are two forces on the 2.00 kg box in the overhead view of Fig. 5-31, but only one is
shown. For F1 = 20.0 N, a = 12.0 m/s2, and θ = 30.0°, find the second force (a) in unit-vector
notation and as (b) a magnitude and (c) an angle relative to the positive direction of the x axis.
Figure 5-31Problem 7.
Answer:
(a) (- 32.0 N) - (20.8 N) ; (b) 38.2 N; (c) - 147°
••8A 2.00 kg object is subjected to three forces that give it an acceleration
. If two of the three forces are
and
find the third force.
••9A 0.340 kg particle moves in an xy plane according to x(t) = -15.00 + 2.00t - 4.00t3 and y(t) =
25.00 + 7.00t - 9.00t2, with x and y in meters and t in seconds. At t = 0.700 s, what are (a) the
magnitude and (b) the angle (relative to the positive direction of the x axis) of the net force on the
particle, and (c) what is the angle of the particle's direction of travel?
Answer:
(a) 8.37 N; (b) - 133°; (c) - 125°
2
3
••10A 0.150 kg particle moves along an x axis according to x(t) = -13.00 + 2.00t + 4.00t - 3.00t , with
x in meters and t in seconds. In unit-vector notation, what is the net force acting on the particle at t
= 3.40 s?
••11A 2.0 kg particle moves along an x axis, being propelled by a variable force directed along that
axis. Its position is given by x = 3.0 m + (4.0 m/s)t + ct2 - (2.0 m/s3)t3, with x in meters and t in
seconds. The factor c is a constant. At t = 3.0 s, the force on the particle has a magnitude of 36 N
and is in the negative direction of the axis. What is c?
Answer:
9.0 m/s2
•••12
Two horizontal forces
and
act on a 4.0 kg disk that slides over frictionless ice, on
which an xy coordinate system is laid out. Force
is in the positive direction of the x axis and
has a magnitude of 7.0 N. Force
has a magnitude of 9.0 N. Figure 5-32 gives the x
component vx of the velocity of the disk as a function of time t during the sliding. What is the
angle between the constant directions of forces
and
?
Figure 5-32Problem 12.
sec. 5-7 Some Particular Forces
•13Figure 5-33 shows an arrangement in which four disks are suspended by cords. The longer, top
cord loops over a frictionless pulley and pulls with a force of magnitude 98 N on the wall to which
it is attached. The tensions in the three shorter cords are T1 = 58.8 N, T2 = 49.0 N, and T3 = 9.8 N.
What are the masses of (a) disk A, (b) disk B, (c) disk C, and (d) disk D?
Figure 5-33Problem 13.
Answer:
(a) 4.0 kg; (b) 1.0 kg; (c) 4.0 kg; (d) 1.0 kg
•14A block with a weight of 3.0 N is at rest on a horizontal surface. A 1.0 N upward force is applied
to the block by means of an attached vertical string. What are the (a) magnitude and (b) direction
of the force of the block on the horizontal surface?
•15
(a) An 11.0 kg salami is supported by a cord that runs to a spring scale, which is supported
by a cord hung from the ceiling (Fig. 5-34a). What is the reading on the scale, which is marked in
weight units? (b) In Fig. 5-34b the salami is supported by a cord that runs around a pulley and to a
scale. The opposite end of the scale is attached by a cord to a wall. What is the reading on the
scale? (c) In Fig. 5-34c the wall has been replaced with a second 11.0 kg salami, and the assembly
is stationary. What is the reading on the scale?
Figure 5-34Problem 15.
Answer:
(a) 108 N; (b) 108 N; (c) 108 N
••16Some insects can walk below a thin rod (such as a twig) by hanging from it. Suppose that such an
insect has mass m and hangs from a horizontal rod as shown in Fig. 5-35, with angle θ = 40°. Its
six legs are all under the same tension, and the leg sections nearest the body are horizontal. (a)
What is the ratio of the tension in each tibia (forepart of a leg) to the insect's weight? (b) If the
insect straightens out its legs somewhat, does the tension in each tibia increase, decrease, or stay
the same?
Figure 5-35Problem 16.
sec. 5-9 Applying Newton's Laws
•17
In Fig. 5-36, let the mass of the block be 8.5 kg and the angle θ be 30°. Find (a) the
tension in the cord and (b) the normal force acting on the block. (c) If the cord is cut, find the
magnitude of the resulting acceleration of the block.
Figure 5-36Problem 17.
Answer:
(a) 42 N; (b) 72 N; (c) 4.9 m/s2
•18
In April 1974, John Massis of Belgium managed to move two passenger railroad cars. He
did so by clamping his teeth down on a bit that was attached to the cars with a rope and then
leaning backward while pressing his feet against the railway ties. The cars together weighed 700
kN (about 80 tons). Assume that he pulled with a constant force that was 2.5 times his body
weight, at an upward angle θ of 30° from the horizontal. His mass was 80 kg, and he moved the
cars by 1.0 m. Neglecting any retarding force from the wheel rotation, find the speed of the cars at
the end of the pull.
•19
A 500 kg rocket sled can be accelerated at a constant rate from rest to 1600 km/h in 1.8 s.
What is the magnitude of the required net force?
Answer:
1.2 × 105 N
•20A car traveling at 53 km/h hits a bridge abutment. A passenger in the car moves forward a distance
of 65 cm (with respect to the road) while being brought to rest by an inflated air bag. What
magnitude of force (assumed constant) acts on the passenger's upper torso, which has a mass of 41
kg?
•21
A constant horizontal force
pushes a 2.00 kg FedEx package across a frictionless floor on
which an xy coordinate system has been drawn. Figure 5-37 gives the package's x and y velocity
components versus time t. What are the (a) magnitude and (b) direction of
?
Figure 5-37Problem 21.
Answer:
(a) 11.7 N; (b) - 59.0°
•22
A customer sits in an amusement park ride in which the compartment is to be pulled
downward in the negative direction of a y axis with an acceleration magnitude of 1.24g, with g =
9.80 m/s2. A 0.567 g coin rests on the customer's knee. Once the motion begins and in unit-vector
notation, what is the coin's acceleration relative to (a) the ground and (b) the customer? (c) How
long does the coin take to reach the compartment ceiling, 2.20 m above the knee? In unit-vector
notation, what are (d) the actual force on the coin and (e) the apparent force according to the
customer's measure of the coin's acceleration?
•23Tarzan, who weighs 820 N, swings from a cliff at the end of a 20.0 m vine that hangs from a high
tree limb and initially makes an angle of 22.0° with the vertical. Assume that an x axis extends
horizontally away from the cliff edge and a y axis extends upward. Immediately after Tarzan steps
off the cliff, the tension in the vine is 760 N. Just then, what are (a) the force on him from the vine
in unit-vector notation and the net force on him (b) in unit-vector notation and as (c) a magnitude
and (d) an angle relative to the positive direction of the x axis? What are the (e) magnitude and (f)
angle of Tarzan's acceleration just then?
Answer:
(a) (285 N) + (705 N) ; (b) (285 N) - (115 N) ; (c) 307 N; (d) - 22.0°; (e) 3.67 m/s2; (f) - 22.0°
•24There are two horizontal forces on the 2.0 kg box in the overhead view of Fig. 5-38 but only one
(of magnitude F1 = 20 N) is shown. The box moves along the x axis. For each of the following
values for the acceleration ax of the box, find the second force in unit-vector notation: (a) 10 m/s2,
(b) 20 m/s2, (c) 0, (d) -10 m/s2, and (e) -20 m/s2.
Figure 5-38Problem 24.
•25Sunjamming. A “sun yacht” is a spacecraft with a large sail that is pushed by sunlight. Although
such a push is tiny in everyday circumstances, it can be large enough to send the spacecraft
outward from the Sun on a cost-free but slow trip. Suppose that the spacecraft has a mass of 900
kg and receives a push of 20 N. (a) What is the magnitude of the resulting acceleration? If the craft
starts from rest, (b) how far will it travel in 1 day and (c) how fast will it then be moving?
Answer:
(a) 0.022 m/s2; (b) 8.3 × 104 km; (c) 1.9 × 103 m/s
•26The tension at which a fishing line snaps is commonly called the line's “strength.”What minimum
strength is needed for a line that is to stop a salmon of weight 85 N in 11 cm if the fish is initially
drifting at 2.8 m/s? Assume a constant deceleration.
•27
An electron with a speed of 1.2 × 107 m/s moves horizontally into a region where a constant
vertical force of 4.5 × 10-16 N acts on it. The mass of the electron is 9.11 × 10-31 kg. Determine the
vertical distance the electron is deflected during the time it has moved 30 mm horizontally.
Answer:
1.5 mm
•28A car that weighs 1.30 × 104 N is initially moving at 40 km/h when the brakes are applied and the
car is brought to a stop in 15 m. Assuming the force that stops the car is constant, find (a) the
magnitude of that force and (b) the time required for the change in speed. If the initial speed is
doubled, and the car experiences the same force during the braking, by what factors are (c) the
stopping distance and (d) the stopping time multiplied? (There could be a lesson here about the
danger of driving at high speeds.)
•29A firefighter who weighs 712 N slides down a vertical pole with an acceleration of 3.00 m/s2,
directed downward. What are the (a) magnitude and (b) direction (up or down) of the vertical force
on the firefighter from the pole and the (c) magnitude and (d) direction of the vertical force on the
pole from the firefighter?
Answer:
(a) 494 N; (b) up; (c) 494 N; (d) down
•30
The high-speed winds around a tornado can drive projectiles into trees, building walls, and
even metal traffic signs. In a laboratory simulation, a standard wood toothpick was shot by
pneumatic gun into an oak branch. The toothpick's mass was 0.13 g, its speed before entering the
branch was 220 m/s, and its penetration depth was 15 mm. If its speed was decreased at a uniform
rate, what was the magnitude of the force of the branch on the toothpick?
••31
A block is projected up a frictionless inclined plane with initial speed v0 = 3.50 m/s.
The angle of incline is θ = 32.0°. (a) How far up the plane does the block go? (b) How long does
it take to get there? (c) What is its speed when it gets back to the bottom?
Answer:
(a) 1.18 m; (b) 0.674 s; (c) 3.50 m/s
••32Figure 5-39 shows an overhead view of a 0.0250 kg lemon half and two of the three horizontal
forces that act on it as it is on a frictionless table. Force
has a magnitude of 6.00 N and is at θ1
= 30.0°. Force
has a magnitude of 7.00 N and is at θ2 = 30.0°. In unit-vector notation, what is
the third force if the lemon half (a) is stationary, (b) has the constant velocity
m/s, and (c) has the varying velocity
m/s2, where t
is time?
Figure 5-39Problem 32.
••33An elevator cab and its load have a combined mass of 1600 kg. Find the tension in the supporting
cable when the cab, originally moving downward at 12 m/s, is brought to rest with constant
acceleration in a distance of 42 m.
Answer:
1.8 × 104 N
••34
In Fig. 5-40, a crate of mass m = 100 kg is pushed at constant speed up a frictionless ramp (θ =
30.0°) by a horizontal force
from the ramp?
. What are the magnitudes of (a)
and (b) the force on the crate
Figure 5-40Problem 34.
••35
The velocity of a 3.00 kg particle is given by
m/s, with time t in
seconds. At the instant the net force on the particle has a magnitude of 35.0 N, what are the
direction (relative to the positive direction of the x axis) of (a) the net force and (b) the particle's
direction of travel?
Answer:
(a) 46.7°; (b) 28.0°
••36Holding on to a towrope moving parallel to a frictionless ski slope, a 50 kg skier is pulled up the
slope, which is at an angle of 8.0° with the horizontal. What is the magnitude Frope of the force on
the skier from the rope when (a) the magnitude v of the skier's velocity is constant at 2.0 m/s and
(b) v = 2.0 m/s as v increases at a rate of 0.10 m/s2?
••37A 40 kg girl and an 8.4 kg sled are on the frictionless ice of a frozen lake, 15 m apart but
connected by a rope of negligible mass. The girl exerts a horizontal 5.2 N force on the rope. What
are the acceleration magnitudes of (a) the sled and (b) the girl? (c) How far from the girl's initial
position do they meet?
Answer:
(a) 0.62 m/s2; (b) 0.13 m/s2; (c) 2.6 m
••38A 40 kg skier skis directly down a frictionless slope angled at 10° to the horizontal. Assume the
skier moves in the negative direction of an x axis along the slope. A wind force with component
Fx acts on the skier. What is Fx if the magnitude of the skier's velocity is (a) constant, (b)
increasing at a rate of 1.0 m/s2, and (c) increasing at a rate of 2.0 m/s2?
••39
A sphere of mass 3.0 × 10-4 kg is suspended from a cord. A steady horizontal breeze pushes
the sphere so that the cord makes a constant angle of 37° with the vertical. Find (a) the push
magnitude and (b) the tension in the cord.
Answer:
(a) 2.2 × 10-3 N; (b) 3.7 × 10-3 N
••40
A dated box of dates, of mass 5.00 kg, is sent sliding up a frictionless ramp at an angle of θ to
the horizontal. Figure 5-41 gives, as a function of time t, the component vx of the box's velocity
along an x axis that extends directly up the ramp. What is the magnitude of the normal force on
the box from the ramp?
Figure 5-41Problem 40.
••41Using a rope that will snap if the tension in it exceeds 387 N, you need to lower a bundle of old
roofing material weighing 449 N from a point 6.1 m above the ground. (a) What magnitude of the
bundle's acceleration will put the rope on the verge of snapping? (b) At that acceleration, with
what speed would the bundle hit the ground?
Answer:
(a) 1.4 m/s2; (b) 4.1 m/s
••42
In earlier days, horses pulled barges down canals in the manner shown in Fig. 5-42. Suppose
the horse pulls on the rope with a force of 7900 N at an angle of θ = 18° to the direction of motion
of the barge, which is headed straight along the positive direction of an x axis. The mass of the
barge is 9500 kg, and the magnitude of its acceleration is 0.12 m/s2. What are the (a) magnitude
and (b) direction (relative to positive x) of the force on the barge from the water?
Figure 5-42Problem 42.
••43
In Fig. 5-43, a chain consisting of five links, each of mass 0.100 kg, is lifted vertically with
constant acceleration of magnitude a = 2.50 m/s2. Find the magnitudes of (a) the force on link 1
from link 2, (b) the force on link 2 from link 3, (c) the force on link 3 from link 4, and (d) the
force on link 4 from link 5. Then find the magnitudes of (e) the force
person lifting the chain and (f) the net force accelerating each link.
on the top link from the
Figure 5-43Problem 43.
Answer:
(a) 1.23 N; (b) 2.46 N; (c) 3.69 N; (d) 4.92 N; (e) 6.15 N; (f) 0.250 N
••44A lamp hangs vertically from a cord in a descending elevator that decelerates at 2.4 m/s2. (a) If the
tension in the cord is 89 N, what is the lamp's mass? (b) What is the cord's tension when the
elevator ascends with an upward acceleration of 2.4 m/s2?
••45An elevator cab that weighs 27.8 kN moves upward. What is the tension in the cable if the cab's
speed is (a) increasing at a rate of 1.22 m/s2 and (b) decreasing at a rate of 1.22 m/s2?
Answer:
(a) 31.3 kN; (b) 24.3 kN
••46An elevator cab is pulled upward by a cable. The cab and its single occupant have a combined
mass of 2000 kg. When that occupant drops a coin, its acceleration relative to the cab is 8.00 m/s2
downward. What is the tension in the cable?
••47
The Zacchini family was renowned for their human-cannonball act in which a family
member was shot from a cannon using either elastic bands or compressed air. In one version of
the act, Emanuel Zacchini was shot over three Ferris wheels to land in a net at the same height as
the open end of the cannon and at a range of 69 m. He was propelled inside the barrel for 5.2 m
and launched at an angle of 53°. If his mass was 85 kg and he underwent constant acceleration
inside the barrel, what was the magnitude of the force propelling him? (Hint: Treat the launch as
though it were along a ramp at 53°. Neglect air drag.)
Answer:
6.4 × 103 N
••48
In Fig. 5-44, elevator cabs A and B are connected by a short cable and can be pulled upward or
lowered by the cable above cab A. Cab A has mass 1700 kg; cab B has mass 1300 kg. A 12.0 kg
box of catnip lies on the floor of cab A. The tension in the cable connecting the cabs is 1.91 × 104
N. What is the magnitude of the normal force on the box from the floor?
Figure 5-44Problem 48.
••49In Fig. 5-45, a block of mass m = 5.00 kg is pulled along a horizontal frictionless floor by a cord
that exerts a force of magnitude F = 12.0 N at an angle θ = 25.0°. (a) What is the magnitude of the
block's acceleration? (b) The force magnitude F is slowly increased. What is its value just before
the block is lifted (completely) off the floor? (c) What is the magnitude of the block's acceleration
just before it is lifted (completely) off the floor?
Figure 5-45Problems 49 and 60.
Answer:
(a) 2.18 m/s2; (b) 116 N; (c) 21.0 m/s2
••50
In Fig. 5-46, three ballot boxes are connected by cords, one of which wraps over a pulley
having negligible friction on its axle and negligible mass. The three masses are mA = 30.0 kg, mB
= 40.0 kg, and mC = 10.0 kg. When the assembly is released from rest, (a) what is the tension in
the cord connecting B and C, and (b) how far does A move in the first 0.250 s (assuming it does
not reach the pulley)?
Figure 5-46Problem 50.
••51
Figure 5-47 shows two blocks connected by a cord (of negligible mass) that passes over a
frictionless pulley (also of negligible mass). The arrangement is known as Atwood's machine. One
block has mass m1 = 1.30 kg; the other has mass m2 = 2.80 kg. What are (a) the magnitude of the
blocks' acceleration and (b) the tension in the cord?
Figure 5-47Problems 51 and 65.
Answer:
(a) 3.6 m/s2; (b) 17 N
••52An 85 kg man lowers himself to the ground from a height of 10.0 m by holding onto a rope that
runs over a frictionless pulley to a 65 kg sandbag. With what speed does the man hit the ground if
he started from rest?
••53In Fig. 5-48, three connected blocks are pulled to the right on a horizontal frictionless table by a
force of magnitude T3 = 65.0 N. If m1 = 12.0 kg, m2 = 24.0 kg, and m3 = 31.0 kg, calculate (a) the
magnitude of the system's acceleration, (b) the tension T1, and (c) the tension T2.
Figure 5-48Problem 53.
Answer:
(a) 0.970 m/s2; (b) 11.6 N; (c) 34.9 N
••54
Figure 5-49 shows four penguins that are being playfully pulled along very slippery
(frictionless) ice by a curator. The masses of three penguins and the tension in two of the cords are
m1 = 12 kg, m3 = 15 kg, m4 = 20 kg, T2 = 111 N, and T4 = 222 N. Find the penguin mass m2 that is
not given.
Figure 5-49Problem 54.
••55
Two blocks are in contact on a frictionless table. A horizontal force is applied
to the larger block, as shown in Fig. 5-50. (a) If m1 = 2.3 kg, m2 = 1.2 kg, and F = 3.2 N, find the
mag nitude of the force between the two blocks. (b) Show that if a force of the same magnitude F
is applied to the smaller block but in the opposite direction, the magnitude of the force between
the blocks is 2.1 N, which is not the same value calculated in (a). (c) Explain the difference.
Figure 5-50Problem 55.
Answer:
(a) 1.1 N
••56
In Fig. 5-51a, a constant horizontal force
is applied to block A, which pushes against block B
with a 20.0 N force directed horizontally to the right. In Fig. 5-51b, the same force
is applied
to block B; now block A pushes on block B with a 10.0 N force directed horizontally to the left.
The blocks have a combined mass of 12.0 kg. What are the magnitudes of (a) their acceleration in
Fig. 5-51a and (b) force
?
Figure 5-51Problem 56.
••57
A block of mass m1 = 3.70 kg on a frictionless plane inclined at angle θ = 30.0° is connected
by a cord over a massless, frictionless pulley to a second block of mass m2 = 2.30 kg (Fig. 5-52).
What are (a) the magnitude of the acceleration of each block, (b) the direction of the acceleration
of the hanging block, and (c) the tension in the cord?
Figure 5-52Problem 57.
Answer:
(a) 0.735 m/s2; (b) down; (c) 20.8 N
••58Figure 5-53 shows a man sitting in a bosun's chair that dangles from a massless rope, which runs
over a massless, frictionless pulley and back down to the man's hand. The combined mass of man
and chair is 95.0 kg. With what force magnitude must the man pull on the rope if he is to rise (a)
with a constant velocity and (b) with an upward acceleration of 1.30 m/s2? (Hint: A free-body
diagram can really help.) If the rope on the right extends to the ground and is pulled by a
coworker, with what force magnitude must the co-worker pull for the man to rise (c) with a
constant velocity and (d) with an upward acceleration of 1.30 m/s2? What is the magnitude of the
force on the ceiling from the pulley system in (e) part a, (f) part b, (g) part c, and (h) part d?
Figure 5-53Problem 58.
••59
A 10 kg monkey climbs up a massless rope that runs over a frictionless tree limb and back
down to a 15 kg package on the ground (Fig. 5-54). (a) What is the magnitude of the least
acceleration the monkey must have if it is to lift the package off the ground? If, after the package
has been lifted, the monkey stops its climb and holds onto the rope, what are the (b) magnitude
and (c) direction of the monkey's acceleration and (d) the tension in the rope?
Figure 5-54Problem 59.
Answer:
(a) 4.9 m/s2; (b) 2.0 m/s2; (c) up; (d) 120 N
••60Figure 5-45 shows a 5.00 kg block being pulled along a frictionless floor by a cord that applies a
force of constant magnitude 20.0 N but with an angle θ(t) that varies with time. When angle θ =
25.0°, at what rate is the acceleration of the block changing if (a) θ(t) = (2.00 × 10-2 deg/s)t and
(b) θ(t) = -(2.00 × 10-2 deg/s)t? (Hint: The angle should be in radians.)
••61
A hot-air balloon of mass M is descending vertically with downward acceleration of
magnitude a. How much mass (ballast) must be thrown out to give the balloon an upward
acceleration of magnitude a? Assume that the upward force from the air (the lift) does not change
because of the decrease in mass.
Answer:
2Ma/(a + g)
•••62
In shot putting, many athletes elect to launch the shot at an angle that is smaller than the
theoretical one (about 42°) at which the distance of a projected ball at the same speed and height
is greatest. One reason has to do with the speed the athlete can give the shot during the
acceleration phase of the throw. Assume that a 7.260 kg shot is accelerated along a straight path
of length 1.650 m by a constant applied force of magnitude 380.0 N, starting with an initial speed
of 2.500 m/s (due to the athlete's preliminary motion). What is the shot's speed at the end of the
acceleration phase if the angle between the path and the horizontal is (a) 30.00° and (b) 42.00°?
(Hint: Treat the motion as though it were along a ramp at the given angle.) (c) By what percent is
the launch speed decreased if the athlete increases the angle from 30.00° to 42.00°?
•••63Figure 5-55 gives, as a function of time t, the force component Fx that acts on a 3.00 kg ice block
that can move only along the x axis. At t = 0, the block is moving in the positive direction of the
axis, with a speed of 3.0 m/s. What are its (a) speed and (b) direction of travel at t = 11 s?
Figure 5-55Problem 63.
Answer:
(a) 8.0 m/s; (b) + x
•••64Figure 5-56 shows a box of mass m2 = 1.0 kg on a frictionless plane inclined at angle θ = 30°. It
is connected by a cord of negligible mass to a box of mass m1 = 3.0 kg on a horizontal
frictionless surface. The pulley is frictionless and massless. (a) If the magnitude of horizontal
force
is 2.3 N, what is the tension in the connecting cord? (b) What is the largest value the
magnitude of
may have without the cord becoming slack?
Figure 5-56Problem 64.
•••65Figure 5-47 shows Atwood's machine, in which two containers are connected by a cord (of
negligible mass) passing over a frictionless pulley (also of negligible mass). At time t = 0,
container 1 has mass 1.30 kg and container 2 has mass 2.80 kg, but container 1 is losing mass
(through a leak) at the constant rate of 0.200 kg/s. At what rate is the acceleration magnitude of
the containers changing at (a) t = 0 and (b) t = 3.00 s? (c) When does the acceleration reach its
maximum value?
Answer:
(a) 0.653 m/s3; (b) 0.896 m/s3; (c) 6.50 s
•••66Figure 5-57 shows a section of a cable-car system. The maximum permissible mass of each car
with occupants is 2800 kg. The cars, riding on a support cable, are pulled by a second cable
attached to the support tower on each car. Assume that the cables are taut and inclined at angle θ
= 35°. What is the difference in tension between adjacent sections of pull cable if the cars are at
the maximum permissible mass and are being accelerated up the incline at 0.81 m/s2?
Figure 5-57Problem 66.
•••67Figure 5-58 shows three blocks attached by cords that loop over frictionless pulleys. Block B lies
on a frictionless table; the masses are mA = 6.00 kg, mB = 8.00 kg, and mC = 10.0 kg. When the
blocks are released, what is the tension in the cord at the right?
Figure 5-58Problem 67.
Answer:
81.7 N
•••68
A shot putter launches a 7.260 kg shot by pushing it along a straight line of length 1.650
m and at an angle of 34.10° from the horizontal, accelerating the shot to the launch speed from its
initial speed of 2.500 m/s (which is due to the athlete's preliminary motion). The shot leaves the
hand at a height of 2.110 m and at an angle of 34.10°, and it lands at a horizontal distance of
15.90 m. What is the magnitude of the athlete's average force on the shot during the acceleration
phase? (Hint: Treat the motion during the acceleration phase as though it were along a ramp at
the given angle.)
Additional Problems
69In Fig. 5-59, 4.0 kg block A and 6.0 kg block B are connected by a string of negligible mass. Force
on block A; force
acts on block B. What is the tension in the
string?
Figure 5-59Problem 69.
Answer:
2.4 N
70
An 80 kg man drops to a concrete patio from a window 0.50 m above the patio. He
neglects to bend his knees on landing, taking 2.0 cm to stop. (a) What is his average acceleration
from when his feet first touch the patio to when he stops? (b) What is the magnitude of the average
stopping force exerted on him by the patio?
71
Figure 5-60 shows a box of dirty money (mass m1 = 3.0 kg) on a frictionless plane inclined
at angle θ1 = 30°. The box is connected via a cord of negligible mass to a box of laundered money
(mass m2 = 2.0 kg) on a frictionless plane inclined at angle θ2 = 60°. The pulley is frictionless and
has negligible mass. What is the tension in the cord?
Figure 5-60Problem 71.
Answer:
16 N
72
Three forces act on a particle that moves with unchanging velocity
Two of the forces are
.
and
. What is the third force?
73
In Fig. 5-61, a tin of antioxidants (m1 = 1.0 kg) on a frictionless inclined surface is
connected to a tin of corned beef (m2 = 2.0 kg). The pulley is massless and frictionless. An upward
force of magnitude F = 6.0 N acts on the corned beef tin, which has a downward acceleration of
5.5 m/s2. What are (a) the tension in the connecting cord and (b) angle β?
Figure 5-61Problem 73.
Answer:
(a) 2.6 N; (b) 17°
74The only two forces acting on a body have magnitudes of 20 N and 35 N and directions that differ
by 80°. The resulting acceleration has a magnitude of 20 m/s2. What is the mass of the body?
75Figure 5-62 is an overhead view of a 12 kg tire that is to be pulled by three horizontal ropes. One
rope's force (F1 = 50 N) is indicated. The forces from the other ropes are to be oriented such that
the tire's acceleration magnitude a is least. What is that least a if (a) F2 = 30 N, F3 = 20 N; (b) F2 =
30 N, F3 = 10 N; and (c) F2 = F3 = 30 N?
Figure 5-62Problem 75.
Answer:
(a) 0; (b) 0.83 m/s2; (c) 0
76A block of mass M is pulled along a horizontal frictionless surface by a rope of mass m, as shown
in Fig. 5-63. A horizontal force acts on one end of the rope. (a) Show that the rope must sag,
even if only by an imperceptible amount. Then, assuming that the sag is negligible, find (b) the
acceleration of rope and block, (c) the force on the block from the rope, and (d) the tension in the
rope at its midpoint.
Figure 5-63Problem 76.
77
A worker drags a crate across a factory floor by pulling on a rope tied to the crate. The
worker exerts a force of magnitude F = 450 N on the rope, which is inclined at an upward angle θ
= 38° to the horizontal, and the floor exerts a horizontal force of magnitude f = 125 N that opposes
the motion. Calculate the magnitude of the acceleration of the crate if (a) its mass is 310 kg and (b)
its weight is 310 N.
Answer:
(a) 0.74 m/s2; (b) 7.3 m/s2
78
In Fig. 5-64, a force of magnitude 12 N is applied to a FedEx box of mass m2 = 1.0 kg. The
force is directed up a plane tilted by θ = 37°. The box is connected by a cord to a UPS box of mass
m1 = 3.0 kg on the floor. The floor, plane, and pulley are frictionless, and the masses of the pulley
and cord are negligible. What is the tension in the cord?
Figure 5-64Problem 78.
2
79A certain particle has a weight of 22 N at a point where g = 9.8 m/s . What are its (a) weight and
2
(b) mass at a point where g = 4.9 m/s ? What are its (c) weight and (d) mass if it is moved to a
point in space where g = 0?
Answer:
(a) 11 N; (b) 2.2 kg; (c) 0; (d) 2.2 kg
80An 80 kg person is parachuting and experiencing a downward acceleration of 2.5 m/s2. The mass
of the parachute is 5.0 kg. (a) What is the upward force on the open parachute from the air? (b)
What is the downward force on the parachute from the person?
81A spaceship lifts off vertically from the Moon, where g = 1.6 m/s2. If the ship has an upward
acceleration of 1.0 m/s2 as it lifts off, what is the magnitude of the force exerted by the ship on its
pilot, who weighs 735 N on Earth?
Answer:
195 N
82In the overhead view of Fig. 5-65, five forces pull on a box of mass m = 4.0 kg. The force
magnitudes are F1 = 11 N, F2 = 17 N, F3 = 3.0 N, F4 = 14 N, and F5 = 5.0 N, and angle θ4 is 30°.
Find the box's acceleration (a) in unit-vector notation and as (b) a magnitude and (c) an angle
relative to the positive direction of the x axis.
Figure 5-65Problem 82.
83
A certain force gives an object of mass m1 an acceleration of 12.0 m/s2 and an object of
mass m2 an acceleration of 3.30 m/s2. What acceleration would the force give to an object of mass
(a) m2 - m1 and (b) m2 + m1?
Answer:
(a) 4.6 m/s2; (b) 2.6 m/s2
84
You pull a short refrigerator with a constant force
across a greased (frictionless) floor, either
with horizontal (case 1) or with tilted upward at an angle θ (case 2). (a) What is the ratio of
the refrigerator's speed in case 2 to its speed in case 1 if you pull for a certain time t? (b) What is
this ratio if you pull for a certain distance d?
85A 52 kg circus performer is to slide down a rope that will break if the tension exceeds 425 N. (a)
What happens if the performer hangs stationary on the rope? (b) At what magnitude of
acceleration does the performer just avoid breaking the rope?
Answer:
(a) rope breaks; (b) 1.6 m/s2
2
86Compute the weight of a 75 kg space ranger (a) on Earth, (b) on Mars, where g = 3.7 m/s , and (c)
in interplanetary space, where g = 0. (d) What is the ranger's mass at each location?
87An object is hung from a spring balance attached to the ceiling of an elevator cab. The balance
reads 65 N when the cab is standing still. What is the reading when the cab is moving upward (a)
with a constant speed of 7.6 m/s and (b) with a speed of 7.6 m/s while decelerating at a rate of 2.4
m/s2?
Answer:
(a) 65 N; (b) 49 N
88Imagine a landing craft approaching the surface of Callisto, one of Jupiter's moons. If the engine
provides an upward force (thrust) of 3260 N, the craft descends at constant speed; if the engine
provides only 2200 N, the craft accelerates downward at 0.39 m/s2. (a) What is the weight of the
landing craft in the vicinity of Callisto's surface? (b) What is the mass of the craft? (c) What is the
magnitude of the free-fall acceleration near the surface of Callisto?
89A 1400 kg jet engine is fastened to the fuselage of a passenger jet by just three bolts (this is the
usual practice). Assume that each bolt supports one-third of the load. (a) Calculate the force on
each bolt as the plane waits in line for clearance to take off. (b) During flight, the plane encounters
turbulence, which suddenly imparts an upward vertical acceleration of 2.6 m/s2 to the plane.
Calculate the force on each bolt now.
Answer:
(a) 4.6 × 103 N; (b) 5.8 × 103 N
6
90An interstellar ship has a mass of 1.20 × 10 kg and is initially at rest relative to a star system. (a)
What constant acceleration is needed to bring the ship up to a speed of 0.10c (where c is the speed
of light, 3.0 × 108 m/s) relative to the star system in 3.0 days? (b) What is that acceleration in g
units? (c) What force is required for the acceleration? (d) If the engines are shut down when 0.10c
is reached (the speed then remains constant), how long does the ship take (start to finish) to
journey 5.0 light-months, the distance that light travels in 5.0 months?
91
A motorcycle and 60.0 kg rider accelerate at 3.0 m/s2 up a ramp inclined 10° above the
horizontal. What are the magnitudes of (a) the net force on the rider and (b) the force on the rider
from the motorcycle?
Answer:
(a) 1.8 × 102 N; (b) 6.4 × 102 N
92Compute the initial upward acceleration of a rocket of mass 1.3 × 104 kg if the initial upward force
produced by its engine (the thrust) is 2.6 × 105 N. Do not neglect the gravitational force on the
rocket.
93
Figure 5-66a shows a mobile hanging from a ceiling; it consists of two metal pieces (m1 =
3.5 kg and m2 = 4.5 kg) that are strung together by cords of negligible mass. What is the tension in
(a) the bottom cord and (b) the top cord? Figure 5-66b shows a mobile consisting of three metal
pieces. Two of the masses are m3 = 4.8 kg and m5 = 5.5 kg. The tension in the top cord is 199 N.
What is the tension in (c) the lowest cord and (d) the middle cord?
Figure 5-66Problem 93.
Answer:
(a) 44 N; (b) 78 N; (c) 54 N; (d) 152 N
94For sport, a 12 kg armadillo runs onto a large pond of level, frictionless ice. The armadillo's initial
velocity is 5.0 m/s along the positive direction of an x axis. Take its initial position on the ice as
being the origin. It slips over the ice while being pushed by a wind with a force of 17 N in the
positive direction of the y axis. In unit-vector notation, what are the animal's (a) velocity and (b)
position vector when it has slid for 3.0 s?
95Suppose that in Fig. 5-12, the masses of the blocks are 2.0 kg and 4.0 kg. (a) Which mass should
the hanging block have if the magnitude of the acceleration is to be as large as possible? What then
are (b) the magnitude of the acceleration and (c) the tension in the cord?
Answer:
(a) 4 kg; (b) 6.5 m/s2; (c) 13 N
96A nucleus that captures a stray neutron must bring the neutron to a stop within the diameter of the
nucleus by means of the strong force. That force, which “glues” the nucleus together, is
approximately zero outside the nucleus. Suppose that a stray neutron with an initial speed of 1.4 ×
107 m/s is just barely captured by a nucleus with diameter d = 1.0 × 10-14 m. Assuming the strong
force on the neutron is constant, find the magnitude of that force. The neutron's mass is 1.67 × 1027
kg.
Copyright © 2011 John Wiley & Sons, Inc. All rights reserved.
PROBLEMS
sec. 6-3 Properties of Friction
•1The floor of a railroad flatcar is loaded with loose crates having a coefficient of static friction of
0.25 with the floor. If the train is initially moving at a speed of 48 km/h, in how short a distance
can the train be stopped at constant acceleration without causing the crates to slide over the floor?
Answer:
36 m
•2In a pickup game of dorm shuffleboard, students crazed by final exams use a broom to propel a
calculus book along the dorm hallway. If the 3.5 kg book is pushed from rest through a distance of
0.90 m by the horizontal 25 N force from the broom and then has a speed of 1.60 m/s, what is the
coefficient of kinetic friction between the book and floor?
•3
A bedroom bureau with a mass of 45 kg, including drawers and clothing, rests on the
floor. (a) If the coefficient of static friction between the bureau and the floor is 0.45, what is the
magnitude of the minimum horizontal force that a person must apply to start the bureau moving?
(b) If the drawers and clothing, with 17 kg mass, are removed before the bureau is pushed, what is
the new minimum magnitude?
Answer:
(a) 2.0 × 102 N; (b) 1.2 × 102 N
•4A slide-loving pig slides down a certain 35° slide in twice the time it would take to slide down a
frictionless 35° slide. What is the coefficient of kinetic friction between the pig and the slide?
•5
A 2.5 kg block is initially at rest on a horizontal surface. A horizontal force
of magnitude 6.0
N and a vertical force are then applied to the block (Fig. 6-17). The coefficients of friction for
the block and surface are μs = 0.40 and μk = 0.25. Determine the magnitude of the frictional force
acting on the block if the magnitude of
is (a) 8.0 N, (b) 10 N, and (c) 12 N.
Figure 6-17Problem 5.
Answer:
(a) 6.0 N; (b) 3.6 N; (c) 3.1 N
•6A baseball player with mass m = 79 kg, sliding into second base, is retarded by a frictional force of
magnitude 470 N. What is the coefficient of kinetic friction μk between the player and the ground?
•7
A person pushes horizontally with a force of 220 N on a 55 kg crate to move it across a
level floor. The coefficient of kinetic friction is 0.35. What is the magnitude of (a) the frictional
force and (b) the crate's acceleration?
Answer:
(a) 1.9 × 102 N; (b) 0.56 m/s2
•8
The mysterious sliding stones. Along the remote Racetrack Playa in Death Valley,
California, stones sometimes gouge out prominent trails in the desert floor, as if the stones had
been migrating (Fig. 6-18). For years curiosity mounted about why the stones moved. One
explanation was that strong winds during occasional rainstorms would drag the rough stones over
ground softened by rain. When the desert dried out, the trails behind the stones were hard-baked in
place. According to measurements, the coefficient of kinetic friction between the stones and the
wet playa ground is about 0.80. What horizontal force must act on a 20 kg stone (a typical mass) to
maintain the stone's motion once a gust has started it moving? (Story continues with Problem 37.)
Figure 6-18Problem 8. What moved the stone?
(Jerry Schad/ Photo Researchers)
•9
A 3.5 kg block is pushed along a horizontal floor by a force of magnitude 15 N at an angle θ
= 40° with the horizontal (Fig. 6-19). The coefficient of kinetic friction between the block and the
floor is 0.25. Calculate the magnitudes of (a) the frictional force on the block from the floor and
(b) the block's acceleration.
Figure 6-19Problems 9 and 32.
Answer:
(a) 11 N; (b) 0.14 m/s2
•10Figure 6-20 shows an initially stationary block of mass m on a floor. A force of magnitude
0.500mg is then applied at upward angle θ = 20°. What is the magnitude of the acceleration of the
block across the floor if the friction coefficients are (a) μs = 0.600 and μk = 0.500 and (b) μs =
0.400 and μk = 0.300?
Figure 6-20Problem 10.
•11
A 68 kg crate is dragged across a floor by pulling on a rope attached to the crate and
inclined 15° above the horizontal. (a) If the coefficient of static friction is 0.50, what minimum
force magnitude is required from the rope to start the crate moving? (b) If μk = 0.35, what is the
magnitude of the initial acceleration of the crate?
Answer:
(a) 3.0 × 102 N; (b) 1.3 m/s2
•12In about 1915, Henry Sincosky of Philadelphia suspended himself from a rafter by gripping the
rafter with the thumb of each hand on one side and the fingers on the opposite side (Fig. 6-21).
Sincosky's mass was 79 kg. If the coefficient of static friction between hand and rafter was 0.70,
what was the least magnitude of the normal force on the rafter from each thumb or opposite
fingers? (After suspending himself, Sincosky chinned himself on the rafter and then moved handover-hand along the rafter. If you do not think Sincosky's grip was remarkable, try to repeat his
stunt.)
Figure 6-21Problem 12.
•13A worker pushes horizontally on a 35 kg crate with a force of magnitude 110 N. The coefficient of
static friction between the crate and the floor is 0.37. (a) What is the value of fs,max under the
circumstances? (b) Does the crate move? (c) What is the frictional force on the crate from the
floor? (d) Suppose, next, that a second worker pulls directly upward on the crate to help out. What
is the least vertical pull that will allow the first worker's 110 N push to move the crate? (e) If,
instead, the second worker pulls horizontally to help out, what is the least pull that will get the
crate moving?
Answer:
(a) 1.3 × 102 N; (b) no; (c) 1.1 × 102 N; (d) 46 N; (e) 17 N
•14Figure 6-22 shows the cross section of a road cut into the side of a mountain. The solid line AA′
represents a weak bedding plane along which sliding is possible. Block B directly above the
highway is separated from uphill rock by a large crack (called a joint), so that only friction
between the block and the bedding plane prevents sliding. The mass of the block is 1.8 × 107 kg,
the dip angle θ of the bedding plane is 24°, and the coefficient of static friction between block and
plane is 0.63. (a) Show that the block will not slide under these circumstances. (b) Next, water
seeps into the joint and expands upon freezing, exerting on the block a force parallel to AA′.
What minimum value of force magnitude F will trigger a slide down the plane?
Figure 6-22Problem 14.
•15The coefficient of static friction between Teflon and scrambled eggs is about 0.04. What is the
smallest angle from the horizontal that will cause the eggs to slide across the bottom of a Tefloncoated skillet?
Answer:
2°
••16A loaded penguin sled weighing 80 N rests on a plane inclined at angle θ = 20° to the horizontal
(Fig. 6-23). Between the sled and the plane, the coefficient of static friction is 0.25, and the
coefficient of kinetic friction is 0.15. (a) What is the least magnitude of the force parallel to the
plane, that will prevent the sled from slipping down the plane? (b) What is the minimum
magnitude F that will start the sled moving up the plane? (c) What value of F is required to move
the sled up the plane at constant velocity?
Figure 6-23Problems 16 and 22.
••17
In Fig. 6-24, a force acts on a block weighing 45 N. The block is initially at rest on a plane
inclined at angle θ = 15° to the horizontal. The positive direction of the x axis is up the plane. The
coefficients of friction between block and plane are μs = 0.50 and μk = 0.34. In unit-vector
notation, what is the frictional force on the block from the plane when
N) , and (c) (- 15 N) ?
is (a) (- 5.0 N) , (b) (- 8.0
Figure 6-24Problem 17.
Answer:
(a) (17 N) ; (b) (20 N) ; (c) (15 N)
••18You testify as an expert witness in a case involving an accident in which car A slid into the rear of
car B, which was stopped at a red light along a road headed down a hill (Fig. 6-25). You find that
the slope of the hill is θ = 12.0°, that the cars were separated by distance d = 24.0 m when the
driver of car A put the car into a slide (it lacked any automatic anti-brake-lock system), and that
the speed of car A at the onset of braking was v0 = 18.0 m/s. With what speed did car A hit car B if
the coefficient of kinetic friction was (a) 0.60 (dry road surface) and (b) 0.10 (road surface
covered with wet leaves)?
Figure 6-25Problem 18.
••19
A 12 N horizontal force pushes a block weighing 5.0 N against a vertical wall (Fig. 6-26). The
coefficient of static friction between the wall and the block is 0.60, and the coefficient of kinetic
friction is 0.40. Assume that the block is not moving initially. (a) Will the block move? (b) In
unit-vector notation, what is the force on the block from the wall?
Figure 6-26Problem 19.
Answer:
(a) no; (b) (- 12 N) + (5.0 N)
••20
In Fig. 6-27, a box of Cheerios (mass mC = 1.0 kg) and a box of Wheaties (mass mW = 3.0 kg)
are accelerated across a horizontal surface by a horizontal force
applied to the Cheerios box.
The magnitude of the frictional force on the Cheerios box is 2.0 N, and the magnitude of the
frictional force on the Wheaties box is 4.0 N. If the magnitude of
of the force on the Wheaties box from the Cheerios box?
is 12 N, what is the magnitude
Figure 6-27Problem 20.
••21An initially stationary box of sand is to be pulled across a floor by means of a cable in which the
tension should not exceed 1100 N. The coefficient of static friction between the box and the floor
is 0.35. (a) What should be the angle between the cable and the horizontal in order to pull the
greatest possible amount of sand, and (b) what is the weight of the sand and box in that situation?
Answer:
(a) 19°; (b) 3.3 kN
••22
In Fig. 6-23, a sled is held on an inclined plane by a cord pulling directly up the plane. The
sled is to be on the verge of moving up the plane. In Fig. 6-28, the magnitude F required of the
cord's force on the sled is plotted versus a range of values for the coefficient of static friction μs
between sled and plane: F1 = 2.0 N, F2 = 5.0 N, and μ2 = 0.50. At what angle θ is the plane
inclined?
Figure 6-28Problem 22.
••23When the three blocks in Fig. 6-29 are released from rest, they accelerate with a magnitude of
0.500 m/s2. Block 1 has mass M, block 2 has 2M, and block 3 has 2M. What is the coefficient of
kinetic friction between block 2 and the table?
Figure 6-29Problem 23.
Answer:
0.37
••24A 4.10 kg block is pushed along a floor by a constant applied force that is horizontal and has a
magnitude of 40.0 N. Figure 6-30 gives the block's speed v versus time t as the block moves along
an x axis on the floor. The scale of the figure's vertical axis is set by vs = 5.0 m/s. What is the
coefficient of kinetic friction between the block and the floor?
Figure 6-30Problem 24.
••25
Block B in Fig. 6-31 weighs 711 N. The coefficient of static friction between block
and table is 0.25; angle θ is 30°; assume that the cord between B and the knot is horizontal. Find
the maximum weight of block A for which the system will be stationary.
Figure 6-31Problem 25.
Answer:
1.0 × 102 N
••26
Figure 6-32 shows three crates being pushed over a concrete floor by a horizontal force of
magnitude 440 N. The masses of the crates are m1 = 30.0 kg, m2 = 10.0 kg, and m3 = 20.0 kg. The
coefficient of kinetic friction between the floor and each of the crates is 0.700. (a) What is the
magnitude F32 of the force on crate 3 from crate 2? (b) If the crates then slide onto a polished
floor, where the coefficient of kinetic friction is less than 0.700, is magnitude F32 more than, less
than, or the same as it was when the coefficient was 0.700?
Figure 6-32Problem 26.
••27Body A in Fig. 6-33 weighs 102 N, and body B weighs 32 N. The coefficients of friction between
A and the incline are μs = 0.56 and μk = 0.25. Angle θ is 40°. Let the positive direction of an x axis
be up the incline. In unit-vector notation, what is the acceleration of A if A is initially (a) at rest,
(b) moving up the incline, and (c) moving down the incline?
Figure 6-33Problems 27 and 28.
Answer:
(a) 0; (b) (- 3.9 m/s2) ; (c) (- 1.0 m/s2)
••28In Fig. 6-33, two blocks are connected over a pulley. The mass of block A is 10 kg, and the
coefficient of kinetic friction between A and the incline is 0.20. Angle θ of the incline is 30°.
Block A slides down the incline at constant speed. What is the mass of block B?
••29In Fig. 6-34, blocks A and B have weights of 44 N and 22 N, respectively. (a) Determine the
minimum weight of block C to keep A from sliding if μs between A and the table is 0.20. (b) Block
C suddenly is lifted off A. What is the acceleration of block A if μk between A and the table is
0.15?
Figure 6-34Problem 29.
Answer:
(a) 66 N; (b) 2.3 m/s2
••30A toy chest and its contents have a combined weight of 180 N. The coefficient of static friction
between toy chest and floor is 0.42. The child in Fig. 6-35 attempts to move the chest across the
floor by pulling on an attached rope. (a) If θ is 42°, what is the magnitude of the force that the
child must exert on the rope to put the chest on the verge of moving? (b) Write an expression for
the magnitude F required to put the chest on the verge of moving as a function of the angle θ.
Determine (c) the value of θ for which F is a minimum and (d) that minimum magnitude.
Figure 6-35Problem 30.
••31
Two blocks, of weights 3.6 N and 7.2 N, are connected by a massless string and slide down
a 30° inclined plane. The coefficient of kinetic friction between the lighter block and the plane is
0.10, and the coefficient between the heavier block and the plane is 0.20. Assuming that the
lighter block leads, find (a) the magnitude of the acceleration of the blocks and (b) the tension in
the taut string.
Answer:
(a) 3.5 m/s2; (b) 0.21 N
••32
A block is pushed across a floor by a constant force that is applied at downward angle θ (Fig.
6-19). Figure 6-36 gives the acceleration magnitude a versus a range of values for the coefficient
of kinetic friction μk between block and floor: a1 = 3.0 m/s2, μk2 = 0.20, and μk3 = 0.40. What is the
value of θ?
Figure 6-36Problem 32.
•••33
A 1000 kg boat is traveling at 90 km/h when its engine is shut off. The magnitude of the
frictional force
between boat and water is proportional to the speed v of the boat: fk = 70v,
where v is in meters per second and fk is in newtons. Find the time required for the boat to slow to
45 km/h.
Answer:
9.9 s
•••34
In Fig. 6-37, a slab of mass m1 = 40 kg rests on a frictionless floor, and a block of mass m2 =
10 kg rests on top of the slab. Between block and slab, the coefficient of static friction is 0.60,
and the coefficient of kinetic friction is 0.40. A horizontal force
of magnitude 100 N begins to
pull directly on the block, as shown. In unit-vector notation, what are the resulting accelerations
of (a) the block and (b) the slab?
Figure 6-37Problem 34.
•••35
The two blocks (m = 16 kg and M = 88 kg) in Fig. 6-38 are not attached to each other. The
coefficient of static friction between the blocks is μs = 0.38, but the surface beneath the larger
block is frictionless. What is the minimum magnitude of the horizontal force
the smaller block from slipping down the larger block?
required to keep
Figure 6-38Problem 35.
Answer:
4.9 × 102 N
sec. 6-4 The Drag Force and Terminal Speed
•36The terminal speed of a sky diver is 160 km/h in the spread-eagle position and 310 km/h in the
nosedive position. Assuming that the diver's drag coefficient C does not change from one position
to the other, find the ratio of the effective cross-sectional area A in the slower position to that in the
faster position.
••37
Continuation of Problem 8. Now assume that Eq. 6-14 gives the magnitude of the air drag
force on the typical 20 kg stone, which presents to the wind a vertical cross-sectional area of
0.040 m2 and has a drag coefficient C of 0.80. Take the air density to be 1.21 kg/m3, and the
coefficient of kinetic friction to be 0.80. (a) In kilometers per hour, what wind speed V along the
ground is needed to maintain the stone's motion once it has started moving? Because winds along
the ground are retarded by the ground, the wind speeds reported for storms are often measured at a
height of 10 m. Assume wind speeds are 2.00 times those along the ground. (b) For your answer
to (a), what wind speed would be reported for the storm? (c) Is that value reasonable for a highspeed wind in a storm? (Story continues with Problem 65.)
Answer:
(a) 3.2 × 102 km/h; (b) 6.5 × 102 km/h; (c) no
••38Assume Eq. 6-14 gives the drag force on a pilot plus ejection seat just after they are ejected from a
plane traveling horizontally at 1300 km/h. Assume also that the mass of the seat is equal to the
mass of the pilot and that the drag coefficient is that of a sky diver. Making a reasonable guess of
the pilot's mass and using the appropriate vt value from Table 6-1, estimate the magnitudes of (a)
the drag force on the pilot + seat and (b) their horizontal deceleration (in terms of g), both just
after ejection. (The result of (a) should indicate an engineering requirement: The seat must include
a protective barrier to deflect the initial wind blast away from the pilot's head.)
••39Calculate the ratio of the drag force on a jet flying at 1000 km/h at an altitude of 10 km to the drag
force on a prop-driven transport flying at half that speed and altitude. The density of air is 0.38
kg/m3 at 10 km and 0.67 kg/m3 at 5.0 km. Assume that the airplanes have the same effective
cross-sectional area and drag coefficient C.
••40
In downhill speed skiing a skier is retarded by both the air drag force on the body and the
kinetic frictional force on the skis. (a) Suppose the slope angle is θ = 40.0°, the snow is dry snow
with a coefficient of kinetic friction μk = 0.0400, the mass of the skier and equipment is m = 85.0
kg, the cross-sectional area of the (tucked) skier is A = 1.30 m2, the drag coefficient is C = 0.150,
and the air density is 1.20 kg/m3. (a) What is the terminal speed? (b) If a skier can vary C by a
slight amount dC by adjusting, say, the hand positions, what is the corresponding variation in the
terminal speed?
Answer:
2.3
sec. 6-5 Uniform Circular Motion
•41A cat dozes on a stationary merry-go-round, at a radius of 5.4 m from the center of the ride. Then
the operator turns on the ride and brings it up to its proper turning rate of one complete rotation
every 6.0 s. What is the least coefficient of static friction between the cat and the merry-go-round
that will allow the cat to stay in place, without sliding?
Answer:
0.60
•42Suppose the coefficient of static friction between the road and the tires on a car is 0.60 and the car
has no negative lift. What speed will put the car on the verge of sliding as it rounds a level curve of
30.5 m radius?
•43
What is the smallest radius of an unbanked (flat) track around which a bicyclist can travel if
her speed is 29 km/h and the μs between tires and track is 0.32?
Answer:
21 m
•44During an Olympic bobsled run, the Jamaican team makes a turn of radius 7.6 m at a speed of 96.6
km/h. What is their acceleration in terms of g?
••45
A student of weight 667 N rides a steadily rotating Ferris wheel (the student
sits upright). At the highest point, the magnitude of the normal force
on the student from the
seat is 556 N. (a) Does the student feel “light” or “heavy” there? (b) What is the magnitude of
at the lowest point? If the wheel's speed is doubled, what is the magnitude FN at the (c)
highest and (d) lowest point?
Answer:
(a) light; (b) 778 N; (c) 223 N; (d) 1.11 kN
••46A police officer in hot pursuit drives her car through a circular turn of radius 300 m with a
constant speed of 80.0 km/h. Her mass is 55.0 kg. What are (a) the magnitude and (b) the angle
(relative to vertical) of the net force of the officer on the car seat? (Hint: Consider both horizontal
and vertical forces.)
••47
A circular-motion addict of mass 80 kg rides a Ferris wheel around in a vertical circle of
radius 10 m at a constant speed of 6.1 m/s. (a) What is the period of the motion? What is the
magnitude of the normal force on the addict from the seat when both go through (b) the highest
point of the circular path and (c) the lowest point?
Answer:
(a) 10 s; (b) 4.9 × 102 N; (c) 1.1 × 103 N
••48
A roller-coaster car has a mass of 1200 kg when fully loaded with passengers. As the car
passes over the top of a circular hill of radius 18 m, its speed is not changing. At the top of the
hill, what are the (a) magnitude FN and (b) direction (up or down) of the normal force on the car
from the track if the car's speed is v = 11 m/s? What are (c) FN and (d) the direction if v = 14 m/s?
••49In Fig. 6-39, a car is driven at constant speed over a circular hill and then into a circular valley
with the same radius. At the top of the hill, the normal force on the driver from the car seat is 0.
The driver's mass is 70.0 kg. What is the magnitude of the normal force on the driver from the
seat when the car passes through the bottom of the valley?
Figure 6-39Problem 49.
Answer:
1.37 × 103 N
••50An 85.0 kg passenger is made to move along a circular path of radius r = 3.50 m in uniform
circular motion. (a) Figure 6-40a is a plot of the required magnitude F of the net centripetal force
for a range of possible values of the passenger's speed v. What is the plot's slope at v = 8.30 m/s?
(b) Figure 6-40b is a plot of F for a range of possible values of T, the period of the motion. What
is the plot's slope at T = 2.50 s?
Figure 6-40Problem 50.
••51
An airplane is flying in a horizontal circle at a speed of 480 km/h (Fig. 6-41). If its
wings are tilted at angle θ = 40° to the horizontal, what is the radius of the circle in which the
plane is flying? Assume that the required force is provided entirely by an “aerodynamic lift” that
is perpendicular to the wing surface.
Figure 6-41Problem 51.
Answer:
2.2 km
••52
An amusement park ride consists of a car moving in a vertical circle on the end of a rigid
boom of negligible mass. The combined weight of the car and riders is 5.0 kN, and the circle's
radius is 10 m. At the top of the circle, what are the (a) magnitude FB and (b) direction (up or
down) of the force on the car from the boom if the car's speed is v = 5.0 m/s? What are (c) FB and
(d) the direction if v = 12 m/s?
••53An old streetcar rounds a flat corner of radius 9.1 m, at 16 km/h. What angle with the vertical will
be made by the loosely hanging hand straps?
Answer:
12°
••54
In designing circular rides for amusement parks, mechanical engineers must consider how
small variations in certain parameters can alter the net force on a passenger. Consider a passenger
of mass m riding around a horizontal circle of radius r at speed v. What is the variation dF in the
net force magnitude for (a) a variation dr in the radius with v held constant, (b) a variation dv in
the speed with r held constant, and (c) a variation dT in the period with r held constant?
••55A bolt is threaded onto one end of a thin horizontal rod, and the rod is then rotated horizontally
about its other end. An engineer monitors the motion by flashing a strobe lamp onto the rod and
bolt, adjusting the strobe rate until the bolt appears to be in the same eight places during each full
rotation of the rod (Fig. 6-42). The strobe rate is 2000 flashes per second; the bolt has mass 30 g
and is at radius 3.5 cm. What is the magnitude of the force on the bolt from the rod?
Figure 6-42Problem 55.
Answer:
2.6 × 103 N
••56
A banked circular highway curve is designed for traffic moving at 60 km/h. The radius of the
curve is 200 m. Traffic is moving along the highway at 40 km/h on a rainy day. What is the
minimum coefficient of friction between tires and road that will allow cars to take the turn without
sliding off the road? (Assume the cars do not have negative lift.)
••57
A puck of mass m = 1.50 kg slides in a circle of radius r = 20.0 cm on a frictionless table while
attached to a hanging cylinder of mass M = 2.50 kg by means of a cord that extends through a
hole in the table (Fig. 6-43). What speed keeps the cylinder at rest?
Figure 6-43Problem 57.
Answer:
1.81 m/s
••58
Brake or turn? Figure 6-44 depicts an overhead view of a car's path as the car travels
toward a wall. Assume that the driver begins to brake the car when the distance to the wall is d =
107 m, and take the car's mass as m = 1400 kg, its initial speed as v0 = 35 m/s, and the coefficient
of static friction as μs = 0.50. Assume that the car's weight is distributed evenly on the four
wheels, even during braking. (a) What magnitude of static friction is needed (between tires and
road) to stop the car just as it reaches the wall? (b) What is the maximum possible static friction
fs,max? (c) If the coefficient of kinetic friction between the (sliding) tires and the road is μk = 0.40,
at what speed will the car hit the wall? To avoid the crash, a driver could elect to turn the car so
that it just barely misses the wall, as shown in the figure. (d) What magnitude of frictional force
would be required to keep the car in a circular path of radius d and at the given speed v0, so that
the car moves in a quarter circle and then parallel to the wall? (e) Is the required force less than
fs,max so that a circular path is possible?
Figure 6-44Problem 58.
•••59
In Fig. 6-45, a 1.34 kg ball is connected by means of two massless strings, each of
length L = 1.70 m, to a vertical, rotating rod. The strings are tied to the rod with separation d =
1.70 m and are taut. The tension in the upper string is 35 N. What are the (a) tension in the lower
string, (b) magnitude of the net force
direction of
on the ball, and (c) speed of the ball? (d) What is the
?
Figure 6-45Problem 59.
Answer:
(a) 8.74 N; (b) 37.9 N; (c) 6.45 m/s; (d) radially inward
Additional Problems
60In Fig. 6-46, a box of ant aunts (total mass m1 = 1.65 kg) and a box of ant uncles (total mass m2 =
3.30 kg) slide down an inclined plane while attached by a massless rod parallel to the plane. The
angle of incline is θ = 30.0°. The coefficient of kinetic friction between the aunt box and the
incline is μ1 = 0.226; that between the uncle box and the incline is μ2 = 0.113. Compute (a) the
tension in the rod and (b) the magnitude of the common acceleration of the two boxes. (c) How
would the answers to (a) and (b) change if the uncles trailed the aunts?
Figure 6-46Problem 60.
61
A block of mass mt = 4.0 kg is put on top of a block of mass mb = 5.0 kg. To cause the top
block to slip on the bottom one while the bottom one is held fixed, a horizontal force of at least 12
N must be applied to the top block. The assembly of blocks is now placed on a horizontal,
frictionless table (Fig. 6-47). Find the magnitudes of (a) the maximum horizontal force that can be
applied to the lower block so that the blocks will move together and (b) the resulting acceleration
of the blocks.
Figure 6-47Problem 61.
Answer:
(a) 27 N; (b) 3.0 m/s2
62A 5.00 kg stone is rubbed across the horizontal ceiling of a cave passageway (Fig. 6-48). If the
coefficient of kinetic friction is 0.65 and the force applied to the stone is angled at θ = 70.0°, what
must the magnitude of the force be for the stone to move at constant velocity?
Figure 6-48Problem 62.
63
In Fig. 6-49, a 49 kg rock climber is climbing a “chimney.” The coefficient of static
friction between her shoes and the rock is 1.2; between her back and the rock is 0.80. She has
reduced her push against the rock until her back and her shoes are on the verge of slipping. (a)
Draw a free-body diagram of her. (b) What is the magnitude of her push against the rock? (c) What
fraction of her weight is supported by the frictional force on her shoes?
Figure 6-49Problem 63.
Answer:
(b) 240 N; (c) 0.60
64A high-speed railway car goes around a flat, horizontal circle of radius 470 m at a constant speed.
The magnitudes of the horizontal and vertical components of the force of the car on a 51.0 kg
passenger are 210 N and 500 N, respectively. (a) What is the magnitude of the net force (of all the
forces) on the passenger? (b) What is the speed of the car?
65
Continuation of Problems 8 and 37. Another explanation is that the stones move only
when the water dumped on the playa during a storm freezes into a large, thin sheet of ice. The
stones are trapped in place in the ice. Then, as air flows across the ice during a wind, the air-drag
forces on the ice and stones move them both, with the stones gouging out the trails. The magnitude
of the air-drag force on this horizontal “ice sail” is given by Dice = 4CiceρAicev2, where Cice is the
drag coefficient (2.0 × 10-3), ρ is the air density (1.21 kg/m3), Aice is the horizontal area of the ice,
and v is the wind speed along the ice.
Assume the following: The ice sheet measures 400 m by 500 m by 4.0 mm and has a coefficient of
kinetic friction of 0.10 with the ground and a density of 917 kg/m3. Also assume that 100 stones
identical to the one in Problem 8 are trapped in the ice. To maintain the motion of the sheet, what
are the required wind speeds (a) near the sheet and (b) at a height of 10 m? (c) Are these
reasonable values for high-speed winds in a storm?
Answer:
(a) 69 km/h; (b) 139 km/h; (c) yes
66
In Fig. 6-50, block 1 of mass m1 = 2.0 kg and block 2 of mass m2 = 3.0 kg are connected by a
string of negligible mass and are initially held in place. Block 2 is on a frictionless surface tilted at
θ = 30°. The coefficient of kinetic friction between block 1 and the horizontal surface is 0.25. The
pulley has negligible mass and friction. Once they are released, the blocks move. What then is the
tension in the string?
Figure 6-50Problem 66.
67In Fig. 6-51, a crate slides down an inclined right-angled trough. The coefficient of kinetic friction
between the crate and the trough is μk. What is the acceleration of the crate in terms of μk, θ, and g?
Figure 6-51Problem 67.
Answer:
g (sin θ - 20.5μk cos θ)
68Engineering a highway curve. If a car goes through a curve too fast, the car tends to slide out of
the curve. For a banked curve with friction, a frictional force acts on a fast car to oppose the
tendency to slide out of the curve; the force is directed down the bank (in the direction water
would drain). Consider a circular curve of radius R = 200 m and bank angle θ, where the
coefficient of static friction between tires and pavement is μs. A car (without negative lift) is driven
around the curve as shown in Fig. 6-11. (a) Find an expression for the car speed vmax that puts the
car on the verge of sliding out. (b) On the same graph, plot vmax versus angle θ for the range 0° to
50°, first for μs = 0.60 (dry pavement) and then for μs = 0.050 (wet or icy pavement). In kilometers
per hour, evaluate vmax for a bank angle of θ = 10° and for (c) μs = 0.60 and (d) μs = 0.050. (Now
you can see why accidents occur in highway curves when icy conditions are not obvious to drivers,
who tend to drive at normal speeds.)
69
A student, crazed by final exams, uses a force of magnitude 80 N and angle θ = 70° to push a
5.0 kg block across the ceiling of his room (Fig. 6-52). If the coefficient of kinetic friction between
the block and the ceiling is 0.40, what is the magnitude of the block's acceleration?
Figure 6-52Problem 69.
Answer:
3.4 m/s2
70Figure 6-53 shows a conical pendulum, in which the bob (the small object at the lower end of the
cord) moves in a horizontal circle at constant speed. (The cord sweeps out a cone as the bob
rotates.) The bob has a mass of 0.040 kg, the string has length L = 0.90 m and negligible mass, and
the bob follows a circular path of circumference 0.94 m. What are (a) the tension in the string and
(b) the period of the motion?
Figure 6-53Problem 70.
71An 8.00 kg block of steel is at rest on a horizontal table. The coefficient of static friction between
the block and the table is 0.450. A force is to be applied to the block. To three significant figures,
what is the magnitude of that applied force if it puts the block on the verge of sliding when the
force is directed (a) horizontally, (b) upward at 60.0° from the horizontal, and (c) downward at
60.0° from the horizontal?
Answer:
(a) 35.3 N; (b) 39.7 N; (c) 320 N
72A box of canned goods slides down a ramp from street level into the basement of a grocery store
with acceleration 0.75 m/s2 directed down the ramp. The ramp makes an angle of 40° with the
horizontal. What is the coefficient of kinetic friction between the box and the ramp?
73In Fig. 6-54, the coefficient of kinetic friction between the block and inclined plane is 0.20, and
angle θ is 60°. What are the (a) magnitude a and (b) direction (up or down the plane) of the block's
acceleration if the block is sliding down the plane? What are (c) a and (d) the direction if the block
is sent sliding up the plane?
Figure 6-54Problem 73.
Answer:
(a) 7.5 m/s2; (b) down; (c) 9.5 m/s2; (d) down
74A 110 g hockey puck sent sliding over ice is stopped in 15 m by the frictional force on it from the
ice. (a) If its initial speed is 6.0 m/s, what is the magnitude of the frictional force? (b) What is the
coefficient of friction between the puck and the ice?
4
75A locomotive accelerates a 25-car train along a level track. Every car has a mass of 5.0 × 10 kg
and is subject to a frictional force f = 250v, where the speed v is in meters per second and the force
f is in newtons. At the instant when the speed of the train is 30 km/h, the magnitude of its
acceleration is 0.20 m/s2. (a) What is the tension in the coupling between the first car and the
locomotive? (b) If this tension is equal to the maximum force the locomotive can exert on the
train, what is the steepest grade up which the locomotive can pull the train at 30 km/h?
Answer:
(a) 3.0 × 105 N; (b) 1.2°
76A house is built on the top of a hill with a nearby slope at angle θ = 45° (Fig. 6-55). An
engineering study indicates that the slope angle should be reduced because the top layers of soil
along the slope might slip past the lower layers. If the coefficient of static friction between two
such layers is 0.5, what is the least angle through which the present slope should be reduced to
prevent slippage?
Figure 6-55Problem 76.
77What is the terminal speed of a 6.00 kg spherical ball that has a radius of 3.00 cm and a drag
coefficient of 1.60? The density of the air through which the ball falls is 1.20 kg/m3.
Answer:
147 m/s
78A student wants to determine the coefficients of static friction and kinetic friction between a box
and a plank. She places the box on the plank and gradually raises one end of the plank. When the
angle of inclination with the horizontal reaches 30°, the box starts to slip, and it then slides 2.5 m
down the plank in 4.0 s at constant acceleration. What are (a) the coefficient of static friction and
(b) the coefficient of kinetic friction between the box and the plank?
79
Block A in Fig. 6-56 has mass mA = 4.0 kg, and block B has mass mB = 2.0 kg. The
coefficient of kinetic friction between block B and the horizontal plane is μk = 0.50. The inclined
plane is frictionless and at angle θ = 30°. The pulley serves only to change the direction of the cord
connecting the blocks. The cord has negligible mass. Find (a) the tension in the cord and (b) the
magnitude of the acceleration of the blocks.
Figure 6-56Problem 79.
Answer:
(a) 13 N; (b) 1.6 m/s2
80Calculate the magnitude of the drag force on a missile 53 cm in diameter cruising at 250 m/s at
low altitude, where the density of air is 1.2 kg/m3. Assume C = 0.75.
81
A bicyclist travels in a circle of radius 25.0 m at a constant speed of 9.00 m/s. The bicycle–
rider mass is 85.0 kg. Calculate the magnitudes of (a) the force of friction on the bicycle from the
road and (b) the net force on the bicycle from the road.
Answer:
(a) 275 N; (b) 877 N
82In Fig. 6-57, a stuntman drives a car (without negative lift) over the top of a hill, the cross section
of which can be approximated by a circle of radius R = 250 m. What is the greatest speed at which
he can drive without the car leaving the road at the top of the hill?
Figure 6-57Problem 82.
83You must push a crate across a floor to a docking bay. The crate weighs 165 N. The coefficient of
static friction between crate and floor is 0.510, and the coefficient of kinetic friction is 0.32. Your
force on the crate is directed horizontally. (a) What magnitude of your push puts the crate on the
verge of sliding? (b) With what magnitude must you then push to keep the crate moving at a
constant velocity? (c) If, instead, you then push with the same magnitude as the answer to (a),
what is the magnitude of the crate's acceleration?
Answer:
(a) 84.2 N; (b) 52.8 N; (c) 1.87 m/s2
84
In Fig. 6-58, force
is applied to a crate of mass m on a floor where the coefficient of static
friction between crate and floor is μs. Angle θ is initially 0° but is gradually increased so that the
force vector rotates clockwise in the figure. During the rotation, the magnitude F of the force is
continuously adjusted so that the crate is always on the verge of sliding. For μs = 0.70, (a) plot the
ratio F/mg versus θ and (b) determine the angle θinf at which the ratio approaches an infinite value.
(c) Does lubricating the floor increase or decrease θinf, or is the value unchanged? (d) What is θinf
for μs = 0.60?
Figure 6-58Problem 84.
85In the early afternoon, a car is parked on a street that runs down a steep hill, at an angle of 35.0°
relative to the horizontal. Just then the coefficient of static friction between the tires and the street
surface is 0.725. Later, after nightfall, a sleet storm hits the area, and the coefficient decreases due
to both the ice and a chemical change in the road surface because of the temperature decrease. By
what percentage must the coefficient decrease if the car is to be in danger of sliding down the
street?
Answer:
3.4%
86
A sling-thrower puts a stone (0.250 kg) in the sling's pouch (0.010 kg) and then begins to
make the stone and pouch move in a vertical circle of radius 0.650 m. The cord between the pouch
and the person's hand has negligible mass and will break when the tension in the cord is 33.0 N or
more. Suppose the sling-thrower could gradually increase the speed of the stone. (a) Will the
breaking occur at the lowest point of the circle or at the highest point? (b) At what speed of the
stone will that breaking occur?
87
A car weighing 10.7 kN and traveling at 13.4 m/s without negative lift attempts to round an
unbanked curve with a radius of 61.0 m. (a) What magnitude of the frictional force on the tires is
required to keep the car on its circular path? (b) If the coefficient of static friction between the tires
and the road is 0.350, is the attempt at taking the curve successful?
Answer:
(a) 3.21 × 103 N; (b) yes
88In Fig. 6-59, block 1 of mass m1 = 2.0 kg and block 2 of mass m2 = 1.0 kg are connected by a
string of negligible mass. Block 2 is pushed by force of magnitude 20 N and angle θ = 35°. The
coefficient of kinetic friction between each block and the horizontal surface is 0.20. What is the
tension in the string?
Figure 6-59Problem 88.
89
A filing cabinet weighing 556 N rests on the floor. The coefficient of static friction between
it and the floor is 0.68, and the coefficient of kinetic friction is 0.56. In four different attempts to
move it, it is pushed with horizontal forces of magnitudes (a) 222 N, (b) 334 N, (c) 445 N, and (d)
556 N. For each attempt, calculate the magnitude of the frictional force on it from the floor. (The
cabinet is initially at rest.) (e) In which of the attempts does the cabinet move?
Answer:
(a) 222 N; (b) 334 N; (c) 311 N; (d) 311 N; (e) c, d
90
In Fig. 6-60, a block weighing 22 N is held at rest against a vertical wall by a horizontal force of
magnitude 60 N. The coefficient of static friction between the wall and the block is 0.55, and the
coefficient of kinetic friction between them is 0.38. In six experiments, a second force is applied
to the block and directed parallel to the wall with these magnitudes and directions: (a) 34 N, up,
(b) 12 N, up, (c) 48 N, up, (d) 62 N, up, (e) 10 N, down, and (f) 18 N, down. In each experiment,
what is the magnitude of the frictional force on the block? In which does the block move (g) up the
wall and (h) down the wall? (i) In which is the frictional force directed down the wall?
Figure 6-60Problem 90.
91
A block slides with constant velocity down an inclined plane that has slope angle θ. The
block is then projected up the same plane with an initial speed v0. (a) How far up the plane will it
move before coming to rest? (b) After the block comes to rest, will it slide down the plane again?
Give an argument to back your answer.
Answer:
(a)
/(4g sin θ); (b) no
92A circular curve of highway is designed for traffic moving at 60 km/h. Assume the traffic consists
of cars without negative lift. (a) If the radius of the curve is 150 m, what is the correct angle of
banking of the road? (b) If the curve were not banked, what would be the minimum coefficient of
friction between tires and road that would keep traffic from skidding out of the turn when traveling
at 60 km/h?
93A 1.5 kg box is initially at rest on a horizontal surface when at t = 0 a horizontal force
(with t in seconds) is applied to the box. The acceleration of the box as a function
of time t is given by
for 0 ≤ t ≤ 2.8 s and
for t > 2.8 s. (a) What is
the coefficient of static friction between the box and the surface? (b) What is the coefficient of
kinetic friction between the box and the surface?
Answer:
(a) 0.34; (b) 0.24
94A child weighing 140 N sits at rest at the top of a playground slide that makes an angle of 25° with
the horizontal. The child keeps from sliding by holding onto the sides of the slide. After letting go
of the sides, the child has a constant acceleration of 0.86 m/s2 (down the slide, of course). (a) What
is the coefficient of kinetic friction between the child and the slide? (b) What maximum and
minimum values for the coefficient of static friction between the child and the slide are consistent
with the information given here?
95
In Fig. 6-61 a fastidious worker pushes directly along the handle of a mop with a force . The
handle is at an angle θ with the vertical, and μs and μk are the coefficients of static and kinetic
friction between the head of the mop and the floor. Ignore the mass of the handle and assume that
all the mop's mass m is in its head. (a) If the mop head moves along the floor with a constant
velocity, then what is F? (b) Show that if θ is less than a certain value θ0, then
along the handle) is unable to move the mop head. Find θ0.
(still directed
Figure 6-61Problem 95.
Answer:
(a) μkmg/(sin θ - μk cos θ); (b) θ0 = tan-1 μs
96A child places a picnic basket on the outer rim of a merry-go-round that has a radius of 4.6 m and
revolves once every 30 s. (a) What is the speed of a point on that rim? (b) What is the lowest value
of the coefficient of static friction between basket and merry-go-round that allows the basket to
stay on the ride?
97
A warehouse worker exerts a constant horizontal force of magnitude 85 N on a 40 kg box
that is initially at rest on the horizontal floor of the warehouse. When the box has moved a distance
of 1.4 m, its speed is 1.0 m/s. What is the coefficient of kinetic friction between the box and the
floor?
Answer:
0.18
98
In Fig. 6-62, a 5.0 kg block is sent sliding up a plane inclined at θ = 37° while a horizontal force
of magnitude 50 N acts on it. The coefficient of kinetic friction between block and plane is 0.30.
What are the (a) magnitude and (b) direction (up or down the plane) of the block's acceleration?
The block's initial speed is 4.0 m/s. (c) How far up the plane does the block go? (d) When it
reaches its highest point, does it remain at rest or slide back down the plane?
Figure 6-62Problem 98.
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